Yes, if you like how it sounds from the driver's seat, and are only considering rears for the rears-eat occupant's whining you mention, then adding rears without the ability to fade them out is going to degrade the front-seat stage.
If you wanted to do a lot of wire routing from the rear deck to the trunk, we can extend the rear speaker wires from the stock locations down to the CleanSweep and then you could use the 6-channel amplifier in place of the 4.
Musicar Certified audio upgrades
OE-Look Enclosures for F30/F32/F22/etc
Add-A-Sub and Full-Bass Packages
Full Cabin 3W and 4W Upgrades
F10 Enclosure preorder- contact us for details!